GB2068996A

GB2068996A - Heat production

Info

Publication number: GB2068996A
Application number: GB8101587A
Authority: GB
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 1980-01-21
Filing date: 1981-01-20
Publication date: 1981-08-19
Also published as: US4344292A; DE3101414C2; GB2068996B; BE887095A; CA1170067A; FR2474151B1; FR2474151A1; DE3101414A1; SE458280B; JPS56116776A; SE8100263L; JPH0217597B2

Description

1

SPECIFICATION Heat production

GB 2 068 996 A 1 U.S. Patent No. 4,089,186 discloses the use of mixtures in heat-pumps, so as to improve their performance by vaporizing and condensing the mixture in accordance with temperature profiles parallel to those of the external fluids with which the heat exchanges are performed, these heat exchanges being of the counter-current type. The mixtures are defined in U.S. Patent No. 4,089,186 as comprising at least two constituents whose composition does not result in an azeotropic mixture. The embodiments disclosed in U.S. Patent No. 4,089,186 relate to cases where heat is recovered in a wide temperature range. For this reason, U.S. Patent No. 4, 089,186 discloses a preferred embodiment according to which the mixture circulated in the heat-pump is condensed in two stages, so as to supply heat in a narrower 10 temperature range than the temperature range at which heat is recovered. On the other hand, in the above embodiment, the mixture condenses in a temperature range which comprises temperatures in excess of 401C.

When heat is recovered in a wide temperature range and the mixture is a binary mixture, the proportions of the two constituents forming the mixture must not be too different. Thus, in the two examples of U.S. Patent No. 4,089,186, the mixture consists of 40% chloro- difluoromethane and 60% 1,1,2-trichloro-1,2,2-trifluoro ethane in one case and 38% propane and 62% normal pentane in the other case.

Many heat-pumps used for heating buildings require other working conditions. As a matter of fact, in many cases, heat is recovered in a relatively narrow temperature range of, for example, 5 to 1 51C. 20 These heat-pumps often operate by recovering heat from a fluid, such as water or air, whose temperature is relatively low, for example between 0 and 201C, and delivering heat to a fluid, such as water or air, whose temperature is also relatively low, for example, between 20 and 401C.

When using such heat-pumps, the conventionally used working fluid is either monochlorodifluoromethane (herein called R-22) ordichlorodifluoromethane (herein called R-12); the critical temperature, which will be hereinafter called t, is 960C for R-22 and 1 120C for R-1 2. As a rule, the higher the boiling temperature and the critical temperature, the more favourable the performance rate; this leads, however, to a high suction rate and thus to a reduced heat capacity for a given compressor. Selecting R-22 and R-1 2 results from a compromise between these two requirements, as concerns the conventional temperatures used in house-heating, the use of R-1 2 being more appropriate to relatively high temperature levels, for example, higher than 501C. These heat pumps are usually operated with halogenated fluids of the type sold under the trade mark "Freon" for safety reasons, thus avoiding inflammable materials such as hydrocarbons or toxic materials such as ammonia.

The present invention is based on the discovery that it is advantageous to use mixtures comprising a basic constituent, which is the same as used when the heat-pump operates with a single compound, 35 for example, R-22 or R-1 2, and a second constituent in limited amount, usually less than 20%, for example 0.5 to 20%, by weight of the mixture. In order that the amount of the second constituent remain low, it is necessary that its critical temperature be quite different from the critical temperature of the basic constituent, the difference between the critical temperatures being at least 20 C degrees.

In accordance with the present invention, there is provided a process for heating a building by 40 means of a heat pump taking heat from a fluid at a temperature between 0 and 20'C and external to the said building and supplying heat at a higher temperature level to the building to be heated, in which the heat pump operates with an azeotrope-free mixture of fluids that comprises a basic constituent and at least one second constituent, the difference between the critical temperature of the basic constituent and the critical temperature of the second constituent being at least 20 C degrees and the molar 45 concentration of the second constituent in the mixture being in the ranqe 0.5 to 20%.

The mixture used in accordance with the invention may be defined as "disym metric", since the proportions of the constituents of the mixture are quite different, and the second constituent of the disymmetric mixture may have a critical temperature lower than the critical temperature of the basic constituent, provided the difference is as stated, but it is preferably higher than the critical temperature 50 of the basic constituent, since the improvement resulting from the use of such a mixture is far better than the improvement obtained in the other case, as shown in the following Example.

EXAMPLE 1

This example refers to a water-to-water heat-pump as shown in the single Figure of the accompanying drawings, in which the mixture is introduced into an evaporator El through the duct 1 and discharged from it through duct 2 in fully vaporized condition. A compressor K, compresses the vapour mixture into duct 3 to be fed into condenser E21 from which it is discharged through duct 4 in fully condensed condition; the mixture is finally expanded through expansion valve D, and recycled to the evaporator. The evaporator and the condenser consist of double-pipe exchangers, through which the heat-exchanging fluids circulate counter-currently.

1 ml/g of water from a groundwater table is fed through duct 5. This water is supplied at 120C and discharged at 4'C through duct 6. Water is heated in the condenser; it is fed through duct 7 at 2 GB 2 068 996 A 2 200C and discharged through duct 8. Its feeding rate is also 1 m3/h.

The heat-pump is first operated with a mixture of R-22, as the basic constituent, and trichlorofluormethane (11-11) whose critical temperature is 1980C, as the second constituent. By varying the RA 1 concentration expressed in % by mole of the mixture, the following results are obtained, as concerns the performance rate (COP), defined as the ratio of the heat power delivered by 5 the heat pump to the electric power consumed by the motor which drives the compressor, and the suction rate (Va) of the compressor, expressed in m3/h.

% mole R-1 1 0 1 5 6 8 COP 3.87 3.97 5.01 5 X 4.60' Va (m'/ h) 9.09 9.16 9.46 9.74 1 O.B8 & 1 1 B 1 - i z A The above results show that the composition of the mixture is optimum for a 6% RA 1 concentration corresponding to a power saving of 23 to 24% with respect to the basic case, this 10 improvement being obtained without modifying the equipment and the exchange surfaces.

There is then used a mixture of R-22 as the basic constituent and chlorotrifluoromethane (11-13), whose critical temperature is 290C, as the second constituent. By varying the RA 3 concentration expressed in % by mole of the mixture, the following results are obtained as concerns the performance rate (COP)a and the suction rate of the compressor (Va) expressed in m'/h.

% mole R-1 3 0 4 8 12. 22; COP 3.87 3.95 4WO 4.04' 4.02 Va (m'lh) 1 9.09 1 8.57 1 8.12- 7.71 6.92 The composition of the mixture passes through an optimum at a 12% RA 3 concentration, corresponding to a power saving of 4% with respect to the basic case.

The above example shows that a mixture comprising as the basic constituent R-22 (tc = 961C) and as the second constituent RA 1 (t, = 1981C) whose critical temperature is higher than the critical 20 temperature of R-22, leads to a power saving which is far greater than that obtained with a mixture comprising as the basic constituent R-22 and as the second constituent RA 3 (t. = 29c1C) whose critical temperature is lower than the critical temperature of R-22. The difference between the critical temperatures, which is at least 201C, must not be excessive and is usually lower than 1 501C.

Useful mixtures according to the invention may be manufactured with a basic constituent which is, 25 for example, chlorodifluoromethane (R-22, tc = 961C), dichlorofluoromethane (RA 2, tc = 1 12'C), bromotrifluoromethane (RA 3 B1, tc = 671C), chloropentafluoroethane (RA 15 tc = 80'C), difluoroethane (RA 52 a, tc = 1 13.5OC) or an azeotrope such as R-502 (tc = 821C), which is an azeotrope of R-22 with RA 15 (48. 8/52.2% by weight), or R-500 (tc = 10 5.51C), which is an azeotrope of RA 2 with R-31 (78.0/22.0% by weight); and a second ' constituent whose critical temperature is higher by at least 200C than the critical temperature of the basic constituent and which is, for example, trichlorofluoromethane (RA 1, te = 198OC), dichforotetrafluoroethane (11-114, tc = 14WC), dichlorohexafluoropropane (R-21 6, tc = 180OC), dichlorofluoromethane (R-2 1, tc = 1 78.5OC), octafluorocyclobutane (C-318, tc = 11 5OC) or an azeotrope such as R-506 (tc = 1420C) which is an azeotrope of R-31 with R-1 14 (55.1/44.9% b.w.).

Specific examples are:

R-22 + RA 1 R-22 + RA 14 RA 15 +RA 14 i RA 2 + RA 1 40 3 GB 2 068 996 A 3 R-12 +R-216 R-502 + R-1 14 As shown by the above example, in each case of use, the optimum value of the molar concentration of the second constituent in the mixture must be selected within the range from 0.5 to 20%; however it must not be selected arbitrarily in order to obtain the full advantages of the invention. 5 A mixture of the above type has the disadvantage, at a given feed rate by weight or by mole, to lead to a suction feed rate somewhat higher than when using a heat-pump operated with a single compound. However, since the compression rate is lower, it is usually possible either to use the same compressor as with a pure substance, or even a compressor of lower cost. It follows therefrom that a heat-pump operated with a mixture of the above type offers many advantages as compared with a heat- 10 pump operated with a single compound. It can, however, be desired to reduce the size of the compressor and thus to reduce the feed rate by volume corresponding to a given feed rate by weight.

It has also been found that the advantages of a greatly improved performance rate can be retained, although decreasing the suction volume of the compressor for a given feed rate by weight or by mole, by using a. mixture comprising at least three constituents, including a basic constituent, for 15 example R-12 or R-22, a second constituent whose critical temperature is higher by at least 201C than the critical temperature of the basic constituent, for example R-1 1, R-1 13 or R-1 14, and a third constituent whose critical temperature is lower than the critical temperature of the basic constituent, for example monochlorotrifluoromethan (R-1 3)e The following example will help to properly select the above mixture.

EXAMPLE 2

There is used the same heat-pump as in example 1 (Figure 1). The water feed rates in the evaporator and the condenser are the same as in example 1; water which releases heat in the evaporator is fed at 121C and discharged at 41C and water which is heated in the condenser is fed at 201C.

There is used a mixture of R-22 as the basic constituent, RA 1 as the second constituent and R-1 3 as the third constituent. A mixture comprising 10% of R-1 3 is prepared and the concentration of R-1 1 is v@ried. The following results are obtained, as concerns the performance rate (COP) and the feed rate at the inlet of the compressor (Va) expressed as m3/h.

% mole R-1 1 0 1 2 3 4 5.

COP 4.G3 4.25. 4J 2 4.10' 4.'03 3.93 Va (m.'/ h) 1 7.91 1 7.65. 9 8.15. 1 8.36 9 8.67 1 9.05.

When using a Mixture of the following composition (molar fractions):

R-22: 0.89 RA 1: 0.01 R-1 3:0.10 -1 a 22% improvement is obtained, as compared with the case where R-22 is used alone.

This improvement is thus similar to that obtained in the optimum case of the first example with a mixture of 94% of R-22 and 6% of R-1 1. When working at the same molar feed rate of the mixture, an improvement of 21 % is obtained, as regards the suction rate, with a mixture of 89% R-22, 1 % R-1 1 and 10% R-1 3, as compared to the suction rate obtained with a mixture of 94% R-22 and 6% R-1 1.

The above example is given by way of illustration, and mixtures of different composition and different nature can be used. A mixture of 3 constituents can only be used if it comprises a basic constituent whose concentration is preferably at least 80% by mole, such as R-22 (te = 96OC), R-1 2 (t, = 11 20C), RA 3 B 1 (t, = 67'C), R-1 15 (te = 80'C), R-1 52 a (t, = 11 3.5OC) or an azeotrope such as R-502 (tc = 820C) or R500 (te = 105.5'C), a second constituent whose critical temperature is higher by at least 200C than the critical temperature of the basic constituent, such as RA 1 (t,: = 1 98IC), R- 45 114 (tc = 1461C), R-21 6 (t, = 180OC), R-21 (tc = 1 78.5OC), C-318 (tc = 11 5IC) or an azeotrope such as R-506 (tc = 1421C), and a third constituent whose critical temperature is lower by preferably at least 200C than the critical temperature of the basic constituent, such as, for example, 4 GB 2 068 996 A chlorotrifluoromethane (R-1 3, tc = 2911C) or trifluoromethane (R-23, te = 25.90C). When the basic constituent is R-22, the third constituent may also be, for example, bromotrifluoromethane (R-13 131, tc = 671C) or the azeotrope R-504 (tr = 661C). The molar concentration of the third constituent in the mixture is between 5 and 20%. This proportion must not be too low in order to take a significant advantage of adding this third constituent; it results therefrom that the difference between the critical temperatures of the basic constituent and the third constituent is preferably lower than 1001 C.

The operating conditions are usually so selected that the pressure of the mixture in the evaporator is higher than the atmospheric pressure and that the pressure of the mixture in the condenser does not attain excessive values, for example, values in excess of 30 bars.

The temperature of the mixture at the outlet of the condenser is usually between 0 and 1 001C. 10 The heat-pumps operated with the above-defined mixtures may be of any type.

The compressor may be, for example, a lubricated piston compressor or a dry piston compressor, a screw-co m pressor or a centrifugal compressor. The exchangers may be, for example, double pipeexchangers, pip/calenderexchangers or plate-exchangers.

The heat power may range, for example, from a few Watts for heat-pumps of the individual 15 heating type to several Megawatts for heat-pumps of the collective heating type.

The present process, based on the use of specific mixtures, is particularly advantageous when heat is taken by allowing the temperature of the external fluid to evolve in a relatively narrow range, preferably lower than 15 C degrees, for example, from 5 to 13 C degrees (difference between the inlet and outlet temperatures for the external fluid).

Claims

1. A process for heating a building by means of a heat pump taking heat from a fluid at a temperature between 0 and 201C and external to the said building and supplying heat at a higher temperature level to the building to be heated, in which the heat pump operates with an azeotrope-free mixture of fluids that comprises a basic constituent and at least one second constituent, the difference 25 between the critical temperature of the basic constituent and the critical temperature of the second constituent being at least 20 C degrees and the molar concentration of the second constituent in the mixture being in the range 0.5 to 20%.

2. A process according to Claim 1, in which the second constituent has a critical temperature higher than the critical temperature of the basic constituent.

3. A process according to Claim 1 or 2, in which the mixture is a mixture of halogenated hydrocarbons.

4. A process according to any one of Claims 1 to 3, in which the basic constituent is one of the following: monochlorodifluoromethane, dichiorodifluoromethane, bromotrifluoromethane, difluoroethane, chloropentafluoroethane, azeotrope R-502 and azeotrope R- 500; and the second constituent is one of the following: trichlorofluoromethane, dichlorotetrafluoroethane, dichlorohexafluoropropane, dichlorofluoromethane, octafluorocyclobutane and azeotrope R-506.

5. A process according to Claim 4, in which the mixture comprises monochlorotrifluoromethane as the basic constituent and trichlorofluoromethane as the second constituent.

6. A process according to Claim 2 or any one of Claims 3 to 5 as dependent on Claim 2, in which 40 the mixture comprises at least one third constituent whose critical temperature is lower than the critical temperature of the basic constituent, the difference between the critical temperature of the third constituent and that of the basic constituent being from 20 to 100 C degrees.

7. A process according to Claim 6, in which the mixture comprises monochlorotrifluoromethane as the basic constituent, trichlorofluoromethane as the second constituent and chlorotrifluoromethane as 45 the third constituent.

8. A process according to Claim 6 or 7, in which the molar concentration of the third constituent in the mixture is from 5 to 20%.

9. A process according to one of Claims 1 to 8, in which heat is recovered from the external fluid by allowing the temperature of the latter to evolve within a temperature range narrower than 15 C 50 degrees.

10. A process accoprding to any one of Claims 1 to 9, in which the temperature of the mixture at the outlet from the condenser is between 0 and 1 001C.

11. A process according to any one of Claims 1 to 10, in which the heat exchange between the mixture of fluids and the external fluid is effected in counter-current.

12. A process according to any one of Claims 1 to 11, in which the heat exchange between the mixture of fluids and the fluid to be heated is effected in counter- current.

13. A process according to any one of Claims 1 to 12, in which heat is delivered to a fluid at a temperature between 20 and 400C.

14. A process according to Claim 1, substantially as hereinbefore described in Example 1 or 2. 60

15. Heat pump when operated according to a process according to any one of Claims 1 to 14.

Printed for Her Majesty's Stationer, Office by the Courier Press. Leamington Spa. 1981. Published by the Patent Office, Southampton Buddings. London, WC2A lAY. from which copies may be obtained.

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